Chain transmission device for controlling the timing system of an internal combustion engine with two overhead camshafts

ABSTRACT

A chain transmission device for controlling the timing system of an internal combustion engine with two overhead camshafts, and having a first chain transmission from the drive shaft to an intermediate shaft, and a second chain transmission from the intermediate shaft to both the camshafts; and wherein the first transmission defines a step-down transmission ratio between the drive shaft and the intermediate shaft; the second transmission defines a step-down transmission ratio between the intermediate shaft and the camshafts; the chain of the second transmission has a pitch smaller than the pitch of the chain of the first transmission; and the intermediate shaft is defined by the drive shaft of a fuel pump of the engine.  
     The present invention relates to a chain transmission device for controlling the timing system of an internal combustion engine with two overhead camshafts.

BACKGROUND OF THE INVENTION

[0001] Single-chain control of both the camshafts of internal combustionengines of the above type poses serious problems in terms of size.

[0002] That is, the distance between the axes of the camshafts is adesign feature which depends on the valve arrangement, and, for reasonsof size, should be as small as possible.

[0003] On the other hand, the drive shaft the camshafts have a specifictransmission ratio (1:2), which means, assuming both camshafts arecontrolled by a single chain controlled by a sprocket on the driveshaft, the diameter of the driven sprockets on the camshafts is twicethe diameter of the drive shaft sprocket.

[0004] As the pitch of the chain is substantially imposed by the powertransmitted, and the number of teeth, and hence the diameter, of thedrive shaft sprocket cannot be reduced beyond a given limit, theresulting diameter of the driven sprockets is fairly large andincompatible with the design distance between the camshaft axes.

[0005] In some known engines, the problem has been partially solvedusing two chain transmissions in series with each other: a firsttransmission from the drive shaft to an idle intermediate shaft, and asecond transmission from the intermediate shaft to the two camshafts.Such a solution, however, is relatively complex and expensive, bycalling for an additional intermediate shaft, which serves no otherpurpose, and for respective supporting means on the engine.

SUMMARY OF THE INVENTION

[0006] It is an object of the present invention to provide a chaintransmission device for an internal combustion engine with two overheadcamshafts, designed to overcome the aforementioned drawbacks in astraightforward, effective, low-cost manner.

[0007] According to the present invention, there is provided a chaintransmission device for controlling the timing system of an internalcombustion engine with two overhead camshafts, and comprising a firstchain transmission from the drive shaft to an intermediate shaft, and asecond chain transmission from the intermediate shaft to both saidcamshafts; wherein said first transmission defines a step-downtransmission ratio between said drive shaft and said intermediate shaft;and said second transmission defines a step-down transmission ratiobetween the intermediate shaft and the camshafts; characterized in thatsaid intermediate shaft is the drive shaft of a fuel pump of saidengine; and in that the chain of said second transmission has a pitchsmaller than the pitch of the chain of said first transmission.

BRIEF DESCRIPTION OF THE DRAWING

[0008] A preferred, non-limiting embodiment of the present inventionwill be described by way of example with reference to the accompanyingdrawing.

DETAILED DESCRIPTION OF THE INVENTION

[0009] Number 1 in the accompanying drawing indicates as a whole a chaintransmission device for controlling the timing system of an internalcombustion engine, in particular a diesel engine, 2.

[0010] Engine 2 comprises a drive shaft 3 of axis A and angular speedω_(M); and two overhead camshafts 4 of speed ω_(C)=½ω_(M) and havingrespective axes B, C parallel to each other and to axis A and separatedby a distance i.

[0011] Engine 2 also comprises an intermediate shaft 5 for driving ahigh-pressure fuel pump 6 forming part of a storage injection system(not shown). Using a storage system allows a certain amount of freedomin selection of the angular speed (ω_(I)) of pump 6, i.e. ofintermediate shaft 5. In such a system, the speed of the pump ispreferably the same as that of the drive shaft, or lower according tothe equation N/2*n_(P), where N is a whole number, and n_(P) the numberof pumping members (normally 3 or 4) of the pump. Such a relationshipensures a whole number of pumping strokes in each engine cycle (tworevolutions of the drive shaft), and hence a known constant phaserelationship between variations in the feed pressure of the injectorsand the points in the engine cycle—predetermined and the same for eachcycle—at which pressure is determined by the electronic injectioncontrol system (as is known, in storage systems, the amount of fuelinjected is controlled electronically by varying the opening time of thepins).

[0012] In the case of three pumping members, pump speed must equal N/6the engine speed, e.g. ⅚.

[0013] Device 1 comprises a first chain transmission 7 interposedbetween drive shaft 3 and intermediate shaft 5; and a second chaintransmission 8 interposed between intermediate shaft 5 and the twocamshafts 4.

[0014] First transmission 7 comprises a drive sprocket 10 fitted todrive shaft 3, a driven sprocket 11 fitted to intermediate shaft 5, anda chain 12 meshing with sprockets 10 and 11; and the number of teeth ofsprockets 10, 11 is such as to achieve, between drive shaft 3 andintermediate shaft 5, a step-down transmission ratio t₁=ω_(I)/ω_(M)ranging between ½ and 1 and, for the reasons given above, convenientlydefined by a fraction N/2*n_(P), e.g. 5:6. By way of example, sprockets10 and 11 may comprise 25 and 30 teeth respectively.

[0015] Second transmission 8 comprises a drive sprocket 15 fitted tointermediate shaft 5, a pair of driven sprockets 16 fitted to respectivecamshafts 4, and one chain 17 meshing with sprocket 15 and bothsprockets 16. The number of teeth of sprockets 15, 16 is such as to givea transmission ratio t₂=ω_(C)/ω_(I), which, together with firsttransmission 7, defines a total step-down ratio 1:2 between drive shaft3 and each of camshafts 4. In the example shown, t₂ equals 3:5, andsprockets 15, 16 may comprise 24 and 40 teeth respectively.

[0016] According to a further characteristic of the present invention,chain 17 may be sized for less power, and therefore have a smallerpitch, than chain 12. By way of example, the two pitches may be 8 and9.525 mm respectively.

[0017] Chains 12 and 17 are guided in the usual way by respective pairsof shoes 19, 20; and one of the shoes in each pair is movable by elasticmeans (not shown) to tension the respective chain.

[0018] The advantages of device 1 according to the teachings of thepresent invention will be clear from the foregoing description.

[0019] Using shaft 5 of pump 6 as an intermediate shaft for bothtransmissions 7, 8 provides for achieving a two-stage step-down ratiowith no need for an idle intermediate shaft which serves no otherpurpose.

[0020] Moreover, the smaller pitch of chain 17 as compared with chain 12provides for reducing the diameter of sprockets 16 of camshafts 4, andhence distance i between the relative axes.

[0021] Clearly, changes may be made to device 1 without, however,departing from the scope of the accompanying Claims. In particular, thetransmission ratios of the individual transmissions may vary.

1. A chain transmission device (1) for controlling the timing system ofan internal combustion engine (2) with two overhead camshafts (4), andcomprising a first chain transmission (7) from the drive shaft (3) to anintermediate shaft (5), and a second chain transmission from theintermediate shaft (5) to both said camshafts (4); wherein said firsttransmission (7) defines a step-down transmission ratio (t₁) betweensaid drive shaft (3) and said intermediate shaft (5); and said secondtransmission (8) defines a step-down transmission ratio (t₂) between theintermediate shaft (5) and the camshafts (4); characterized in that saidintermediate shaft (5) is the drive shaft of a fuel pump (6) of saidengine (2); and in that the chain (17) of said second transmission (8)has a pitch (p₂) smaller than the pitch (p₁) of the chain (12) of saidfirst transmission (7).
 2. A device as claimed in claim 1, characterizedin that said first transmission ratio (t₁) ranges between 1:2 and
 1. 3.A device as claimed in claim 1, characterized in that said pump (6) is ahigh-pressure pump of a storage injection system.
 4. A device as claimedin claim 3, characterized in that said first transmission ratio (t₁) isdefined by a fraction N/2*n_(P), where N is a whole number, and n_(P)the number of pumping members of the pump (6).